Used when pulling in new lines on transmission towers is where you usually see the set up you describe.
Typically near where a road crosses a transmission corridor.
New wires are pulled in by first rigging a series of pullieys and synthetic mostly insulating ropes strung under some tension between the pulleys.
Then attach a new conductor to the tail and of the rope and winch the new conductors in, either one a a time or more often in sets of three or four wires at a time.
'Should a rope break, etc during this operation , no harm the wire coming down in the corridor, as it is not yet energized.
But if it falls crossing a roadway that is a hazard for vehicles.
So the temporary 2 poles with a pole strung between them is the protective measure for the rope break scenario at roads, or when you cross over another power line also.
They sometimes stay up a while because often tower lines are multi circuit supports, and usually you can only do with one circuit down at a time.
And you might have to wait for the next year when a low demand time comes to do the next circuit conductor renewal operation.
